The contact breaker (aka points, or set of points) is a type of electro-mechanical switch used to trigger an ignition pulse (spark) in spark-ignition internal combustion engines. The points may be located behind the flywheel, within a distributor or inside of a separate contact breaker housing, depending on the design of the engine. For our purposes, on mopeds, the points are almost invariably located behind the flywheel.
The term "set of points" (note the plural) derives from the fact that the points are a pair of electrical contacts; a fixed contact and a moving contact. In some cases, the fixed and moving contacts are individually replaceable items. In other cases, both contacts are sold as a single assembly. The fixed contact is almost always common to engine ground, while the moving contact is isolated from ground by insulating sleeves and/or washers.
Points, contact points, contact breaker, breaker points and ignition points are all different terms that refer to the same part of the ignition system. Varying by locality, versions of points are most common among US-English speakers, while versions of breaker may be more familiar to UK or European-English speakers. These terms will be used interchangeably throughout this article.
How Points Work
What is a set of points? Simply put, a set of points is an on/off switch -- a very special on on/off switch.
This "switch" is spring loaded and normally closed. It is opened by a rotating (cam) lobe that rubs against a block on the moving contact, known, appropriately enough, as the rubbing block. Once the cam lobe has passed by, spring pressure returns the points to the closed position. The distance which the points open during each cycle (the "point gap") is controlled by the position of the rubbing block with respect to the rotating cam, and is adjustable. A typical point gap would be 0.35-0.40mm (0.014-0.016"), though this will vary from manufacturer to manufacturer.
To prevent excessive wear of the rubbing block or the cam itself, a small piece of oil-soaked felt is fitted adjacent to the cam. As the cam rotates, it contacts this felt and picks up a film of oil which lubricates the assembly. This oil, which is quickly consumed, must be replenished at regular intervals by applying a few drops to the felt and letting it soak in.
The purpose of the contact breaker is to interrupt the current flowing in the primary circuit of the ignition coil. When this occurs, the collapsing magnetic field in the core of the coil induces a high voltage in the secondary winding of the coil, which has very many more turns. This causes a very large voltage (approximately 12,000 volts) to appear at the coil output for a short period – enough to arc across the electrodes of a spark plug and ignite the fuel mixture in the cylinder.
While at first counter-intuitive, it must be carefully noted that the spark plug fires upon opening, rather than closing of the points.
It is not enough that the points open the correct distance, and do so repeatedly and reliably - they must open at precisely the right point in the cycle, when the piston is in the correct position for ignition of the fuel mixture. The spark must hit shortly before the piston reaches top dead center. This "head start", known as ignition timing advance gives the fuel/air mixture a bit of time to begin burning so that maximum power may be extracted from the burning mixture as the piston travels downward on the power stroke. This concept is known as ignition timing, and it plays a significant role in the operation, performance and reliability of all internal combustion engines.
While timing is covered more thoroughly in its own article, it is worthwhile noting that the point gap setting directly affects ignition timing, and in some cases, is the only method available for adjusting the same. In all cases, a wider point gap increases the timing advance, while a narrower gap decreases (retards) timing advance. Better designs offer independent adjustments for both point gap and timing, though it is still common to observe some interaction between the two adjustments due to the simplicity of the mechanism used.
Points cause trouble in many ways. They get old and worn. They can get oily or wet. Oil and water can conduct electricity where it is not needed and cause loss of power. The wires going to them shake and break. The wires may rub against the inside of the flywheel and ground out. Mounting screws can vibrate loose (rarely) and the block no longer rubs against the cam. The spring can break and the points won’t close. Screws or clips holding the wire to the points can disconnect. Running without a cover over their flywheel can be hazardous because one splash of water can stop the moped.
To make the trouble worse, points are under the flywheel. To clean or adjust the points, you need to be peering in a tiny space (often through holes in the flywheel itself) with a flashlight, a feeler gauge, and maybe two screwdrivers. A lift or a table can hold a ped high enough to make the height better, but many people don’t have that luxury.
Many times, problems with the points are accompanied by problems with the condenser. Consider replacing the condenser when you replace your points. (You're in there anyway ... why not?)
Read Fred's Guide for an explanation of how to clean your points.
So, how do you replace points?
- First, you have to take off the flywheel.
- Disconnect the wires (keeping track of which ones go where).
- Unscrew the screw(s) holding the base of the points in place.
- Install the new points, but don’t tighten up the mounting screws. Add the wires to it and nudge the points over to the crankshaft.
- Turn the engine by hand (aren’t you glad you pulled out the spark plug?) Make sure you are turning it the same direction it runs.
- As you are slowly turning that engine, you can see the cam on the crankshaft start to push the points away. You want the farthest place that the cam will push to be resting against the rubbing block. Gently push the assembly into the cam until the points open.
- Now grab your feeler gauge and get ready to put it in the gap.
- Slightly tighten the assembly mount.
- Open the points by hand and rest the feeler gauge between the round contacts
- With one hand you can use a screwdriver in the notch in the points to move the mounting plate to exactly where you want it. With your other hand, drag the .015 (or your spec.) blade through the contacts, moving the first hand until there is barely a drag on the feeler gauge. With your third hand, tighten down the mounting plate.
- Check your timing. Cover everything up.
- Put your spark plug back in, put away your tools and ride.
If the information for a specific brand deviates from the information above, check here for brand-specific information.
- Flywheel removal: Here is where Honda owners are happy folk. The flywheel is bolted to crankshaft. With simple tools it can be taken off to easily expose the points.
- In step #5, when turning the engine on an Express with a tap starter by hand, it is very important to turn it the correct way. You can have serious trouble if you turn them the wrong way.
- Garelli mopeds are a little bit trickier. Garellis have the cam for the points inside the flywheel so they can’t be set until the flywheel is in place. On step #10, the adjustment must be done with the flywheel in place working through the grooves.
- Motobecanes have a cam that is a separate piece that is mounted on the crankshaft. It must be in the right place or you will be in serious trouble.
- Velosolex have a flywheel key that is integral to the flywheel. Adjustments can be made through the openings in the flywheel. There is no separate timing adjustment, only point gap is adjustable to achieve correct timing. The points must be adjusted so that they just begin to open when the "Rupture" marks on the flywheel and engine line up, rotating the flywheel clockwise. There is no set points gap measurement. When the Rupture marks are lined up, there must be minimal space (thickness of a cigarette paper) between the points. Also, VERY IMPORTANT: NEVER clean the point contacts in a Velosolex by filing. Use only a doubled-over strip of ultra-fine (1200-1500 grit) Wet-o-dry sandpaper, pulled lightly through the closed points, to buff the surfaces. Clean with solvent after buffing to remove any particles. Filing will destroy the precious-metal plating on the contacts and cause odd symptoms or even total ignition failure.
Note: The arrow on the stator plate is not indicating rotation; it is pointing to the Rupture mark. Rotation is clockwise.
- Thanks to 50plus for contributing the first edition of this article and pictures to complement it.